In the context of environmental protection, electric vehicles have become a research hotspot in recent years. Electric vehicles can achieve zero or extremely low emissions in urban transportation, offering significant advantages in the environmental field, and countries around the world are striving to develop them. An electric vehicle mainly consists of three parts: a motor drive system, a battery system, and a vehicle control system. The motor drive system, which directly converts electrical energy into mechanical energy, determines the performance indicators of the electric vehicle. Therefore, the selection of the drive motor is particularly important.
Electric vehicles typically employ a powertrain structure with a centralized electric motor drive.
Advantages of centralized motor drive:
(1) Some transmission devices of internal combustion engine vehicles can be reused and arranged in the original engine compartment, with good inheritance;
(2) An integrated structure of motor, speed reduction mechanism, and even controller can be adopted, which is compact and easy to handle problems such as motor cooling, vibration isolation and electromagnetic interference.
(3) The overall layout of the vehicle is similar to that of the internal combustion engine, and the thermal management, sound insulation and collision safety of the front compartment are similar to or easy to handle as the original vehicle.
Disadvantages of centralized motor drive:
(1) The transmission chain is long and the transmission efficiency is low;
(2) High-speed, high-power motors are usually required, and the performance requirements of the motors are high.
Advantages of distributed motor drives:
(1) Electronic differential control technology is used to achieve different speeds of the inner and outer wheels when turning, and the accuracy is higher;
(2) Eliminating the mechanical differential device helps reduce the weight of the power system, improve transmission efficiency, and reduce transmission noise;
(3) It is conducive to the optimization of the overall vehicle layout and the matching optimization of the vehicle dynamic performance;
(4) It reduces the performance requirements of the motor and has the characteristics of high redundancy and reliability.
Disadvantages of distributed motor drives:
(1) To meet the requirements of coordinated movement of each wheel, the synchronous coordination control of multiple motors is highly demanding;
(2) The distributed installation of motors presents many technical challenges, including structural layout, thermal management, electromagnetic compatibility and vibration control.
Distributed motor drives typically come in two forms: hub motors and wheel-side motors. Wheel-side motors mean that each drive wheel is driven by a separate motor, but the motor is not integrated into the wheel; instead, it is connected to the wheel via a transmission device (such as a driveshaft). Wheel-side motors operate within the sprung mass range, resulting in good vibration isolation performance from the suspension system. However, motors mounted on the vehicle body significantly impact the overall vehicle layout, especially in rear-axle drive configurations. Furthermore, the substantial deformation and movement between the vehicle body and wheels imposes limitations on the universal joint transmission of the driveshaft.
Advantages of hub motors:
Compared to wheel-side motors, hub motors have significant advantages.
(1) The transmission device can be completely omitted, and the overall power utilization efficiency is greatly improved;
(2) The hub motor allows the overall vehicle layout to adopt a flat chassis structure, which greatly improves the interior space and layout freedom;
(3) There are almost no high-power moving parts on the vehicle body, which greatly improves the vibration and noise comfort of the whole vehicle;
(4) The hub motor method facilitates the realization of four-wheel drive, which greatly improves the power performance of the whole vehicle;
(5) The hub motor, as an actuator, takes advantage of its fast and accurate response speed to facilitate the integrated control of vehicle dynamics, including drive-by-wire, brake-by-wire and vehicle dynamics control by wire, thereby improving the active safety of the vehicle.
Key technical issues in hub motor system research
In-wheel motors present new technological challenges, primarily including:
(1) The hub motor system integrates multiple functions such as driving, braking and load bearing, which makes the optimization design difficult; (2) The internal space of the wheel is limited, which requires high power density performance of the motor, making the design difficult.
(3) The integration of the motor with the wheel results in a large unsprung mass, which deteriorates the suspension's vibration isolation performance and affects the vehicle's handling and safety under uneven road conditions. At the same time, the hub motor will bear a large road impact load, and the motor's vibration resistance requirements are stringent;
(4) Under the condition of high load and low speed climbing long slope, the hub motor is prone to overheating and burnout due to insufficient cooling. The heat dissipation and forced cooling of the motor need to be taken seriously.
(5) Water and dirt can easily accumulate in the wheel area, leading to corrosion and damage to the motor, which affects its lifespan and reliability.
Summarize
Driving technology development process
Compared to centralized power drive by electric motors, hub motor technology has significant advantages. It offers greater flexibility in layout and eliminates the need for complex mechanical transmission systems. However, it also has its own notable shortcomings, such as the challenges of sealing, balancing starting current/torque, and differential speed issues with the drive wheels during steering. If these engineering challenges can be resolved, hub motor drive technology will have a promising future in new energy vehicles.
